A flat-bed-type image scanner or copier with which a mechanism of correcting an optical path length in sub-scanning is provided is known, for example, as disclosed in Japanese patent application laid-open No.2-101862(1990). FIG. 1A shows a composition of such conventional image scanner. As shown in FIG. 1A, a reflecting mirror 61 reflects a light reflected on a scanned manuscript 60 in the horizontal direction. Reflecting mirrors 62 and 63, which are integrated with each other, allow the reflected light from the reflecting mirror 61 to be turned in the reverse direction. The reflected light from the reflecting mirror 63 is then transmitted through a focusing lens 64, forming an image on an one-dimensional image sensor 65.
In sub-scanning, the reflecting mirror 61 is parallel moved in the direction as shown by an arrow in FIG. 1A to sequentially scan the manuscript 60. At this time, since the unit composed of the reflecting mirrors 62, 63 is parallel moved by half the distance that the reflecting mirror 61 is parallel moved in the same direction, the optical path length from the scanned manuscript 60 to the one-dimensional image sensor 65 can be constant.
On the other hand, known in a conventional flat-bed-type image scanner or copier are a manner that a unit 66 in which a reflecting mirror, a focusing lens and an one-dimensional image sensor are integrated is, as shown in FIG. 1B, parallel moved to a scanned manuscript 60, and a manner that a unit in which a focusing lens 64 and an one-dimensional image sensor 65 are integrated and to which a light reflected on a scanned manuscript 60 is led through reflecting mirrors 61, 67 and 68 is, as shown in FIG. 1C, parallel roved with the parallel movement of the reflecting mirror 61 to the manuscript 60.
Furthermore, Japanese patent application laid-open No.62-291259(1987) discloses an image scanner in which a reflecting mirror is rotated to scan a manuscript and an one-dimensional image sensor reads the image data and with which a mechanism of correcting an optical path length is provided. In this image scanner, the rotation of the reflecting mirror and the parallel movement of an focusing lens and the one-dimensional image sensor are conducted by using a cam mechanism.
A unit in which the reflecting mirror, focusing lens and one-dimensional image sensor are fixed is pressed against a slide plane of the cam by a spring. The cam is composed of two slide planes, where one is used for the reflecting mirror and the other is used for the unit composed of the focusing lens and the one-dimensional image sensor. The slide planes are designed such that the reflecting mirror is driven by a predetermined angle and the unit of the focusing lens and one-dimensional image sensor is driven by a predetermined distance of parallel movement. Rotating the cam by, for example, a stepping motor, the reflecting mirror and the unit of the focusing lens and one-dimensional image sensor can be simultaneously driven.
Japanese patent application laid-open No.8-7073(1996) discloses another image sensor in which a reflecting mirror is rotated to scan a manuscript and an one-dimensional image sensor reads the image data. However, this image scanner is not provided with an optical path length correcting mechanism by which an optical path length from an one-dimensional image sensor to a reading surface is corrected to be constant. Therefore, the image to be read may have a barrel distortion.
However, in the conventional techniques as shown in FIGS. 1A and 1C, there are problems that the driven units and drive mechanism are complicated and that the entire device becomes thicker since the optical path is turned. Also, in the image scanner shown in FIG. 1B, there are problems that the miniaturization of the driven unit is limited and that, when the driven unit is miniaturized, the optical path length becomes shorter, therefore making the designing of optical system difficult. Furthermore, there is a common problem that the image scanners in FIGS. 1A to 1C require a high torque and a high precision of the drive mechanism.
On the other hand, the optical path length correcting mechanism with the cam as disclosed in Japanese patent application laid-open No.62-291259(1987), since the reflecting mirror and the unit of the focusing lens and one-dimensional image sensor are pressed against the slide plane of the cam by the spring, requires a relatively high torque of a drive means such as a stepping motor. Furthermore, there is a problem that, with the increase in the distance of parallel movement for correcting an optical path length and the increase in the rotation angle of the reflecting mirror, the size of the cam needs to be larger in the two-dimensional directions.